This invention generally relates to alloys which operate as short-circuited galvanic cells to corrode rapidly in electrolyre such as seawater. Such an alloy is suitable as a heat source; as a gas generator; or as a corroding release link.
Sources of heat and hydrogen gas of various types are well known in the art, especially by virtue of earlier already issued United States Patents commonly assigned herewith such as: U.S. Pat. No. 3,884,216 issued May 20, 1975 for ELECTROCHEMICAL ENERGY SOURCE FOR DIVER SUIT HEATING; U.S. Pat. No. 3,942,511 issued Mar. 9, 1976 for SANDWICHED STRUCTURE FOR PRODUCTION OF HEAT AND HYDROGEN GAS; U.S. Pat. No. 3,993,577 issued Nov. 23, 1976 for METHOD FOR PRODUCTION OF HEAT AND HYDROGEN GAS; and, U.S. Pat. No. 4,017,414 issued Apr. 12, 1977 for POWERED METAL SOURCE FOR PRODUCTION OF HEAT AND HYDROGEN GAS.
At least two methods have been employed in the past to achieve high corrosion rates. One is to construct a short-circuited battery-like cell of noble and active metal plates separated by an electrode gap such as disclosed in aforementioned U.S. Pat. No. 3,884,216. Another method is to form a powder by mechanically joining the discrete particles of noble and active powders such as disclosed in aforementioned U.S. Pat. Nos. 3,942,511, 3,993,577 and 4,017,414 where each powder particle is a small galvanic cell.
The battery-like cell has two principal disadvantages: the power output is dependent upon the electrode gap (internal cell resistance) and the resistance in the electrical short circuits (external load) limits the reaction rate. In order to maximize power output, the electrode gap must approach zero. Yet, to sustain the reaction, reaction products must be flushed away from the reacting surfaces. This requires a small initial gap between the plates. The gap creates high internal cell resistance which reduces the power obtainable from the cell. A further decline in power occurs because of the gap increase as the active plate is consumed.
The resistance in the electrical short circuit between the noble and active materials can limit power output. To maximize output, the external short circuit resistance must be minimized. In the battery like configuration the resistance is kept low by providing several relatively short-length paths between the plates. Low resistance spacers are used to maintain the electrode gap. Thus, the electrical resistance is minimized within the configuration and material limits.
In the powdered form where each grain of powder is a small galvanic cell similar to the larger battery-like cell, noble metal particles are mechanically joined to the surface of an active metal particle, as disclosed in aforementioned U.S. Pat. No. 4,017,414. The combination retains the property and identity of each constituent. But each cell will react with itself, so no electrode gap is necessary or exists. The short circuit path length is minimized because the particles are in direct contact. However, the short circuit resistance is not minimized. Electrical resistance between individual particles is a function both of physical proximity and of the oxides that exist on the bond surface between the metal particles (this is also true for the battery-like configuration). Because high resistance surface oxides are present, excellent mechanical contact may not assure intimate electrical contact. Due to the random method of joining the particles and low energy level of the balls used in the milling process in the aforesaid patent, some metal particles may not be paired into micro-cells but may remain free and will not react at all. Also, in this prior art powder form, the internal cell resistance may be minimized but the external or load resistance may be high. Due to the high oxide level on the bond surface, compacting and sintering the powders fabricated by using the prior art teachings will not result in barstock, etc., which has any significant mechanical strength.
A strong mechanical and electrical bond is necessary to provide a rapidly corroding galvanic cell alloy.